Apparatus for and method of determining the liquid coolant level in a nuclear reactor

An instrument is described for measuring liquid level within a liquid containing vessel comprising: an elongated body having a longitudinal axis extends into a liquid containing vessel in a generally vertical direction; difference thermocouples arranged within the elongated body, the difference thermocouple comprising a heated and unheated junction; and electrical heating means formed of electrically conductive materials which comprise high and low heating materials located in alternate zones along the length of the elongate body, the electrical heating means comprising a single slender rod which extends down the length of the elongated body parallel to the longitudinal axis, whereby the heated junction of the difference thermocouple is heated by one zone of high heating material located adjacent to the heated junction, and the unheated junction of each of the difference thermocouple is located adjacent to one zone of low heating material